prompt_store: Remove additional code for /project (#27981)

Found leftover from https://github.com/zed-industries/zed/pull/27660

Release Notes:

- N/A

crates/prompt_store/src/prompts.rs

@@ -77,11 +77,6 @@ pub struct TerminalAssistantPromptContext {
     pub user_prompt: String,
 }
 
-#[derive(Serialize)]
-pub struct ProjectSlashCommandPromptContext {
-    pub context_buffer: String,
-}
-
 pub struct PromptLoadingParams<'a> {
     pub fs: Arc<dyn Fs>,
     pub repo_path: Option<PathBuf>,
@@ -375,14 +370,4 @@ impl PromptBuilder {
     pub fn generate_suggest_edits_prompt(&self) -> Result<String, RenderError> {
         self.handlebars.lock().render("suggest_edits", &())
     }
-
-    pub fn generate_project_slash_command_prompt(
-        &self,
-        context_buffer: String,
-    ) -> Result<String, RenderError> {
-        self.handlebars.lock().render(
-            "project_slash_command",
-            &ProjectSlashCommandPromptContext { context_buffer },
-        )
-    }
 }